Method of molding multi-ply polymeric composites including simultaneous curing of plural layers having different catalyst polymerization initiation temperatures and product thereof

ABSTRACT

This invention pertains to a method to permit the molding of a plurality of plies of polymeric compositions into an integral molded object having a minimum number of internal flaws and, therefore, improved properties. The plies are positioned in contiguous relationship, and the initiation of the polymerization reactions is controlled by selecting catalyst systems which substantially simultaneously initiate polymerization in each of the plies to prevent the development of curing seams at the points of contact between the plies.

TECHNICAL FIELD

This invention pertains to the field of molding compounds.

BACKGROUND OF THE INVENTION

The molding of polymeric composites such as sheet molding compounds andbulk molding compounds is well known. Generally, such polymericcomposites contain glass fiber reinforced polyester-based resins, filledor unfilled, which crosslink by a polymerization reaction which takesplace most usually under heat and pressure. Such sheet molding compoundmaterial is commonly prepared and stored between flexible polymeric filmlayers in the form of plies on rolls or continuously folded into largestorage containers for the duration of the chemical thickening ormaturation period before being molded. In preparation for the moldingprocess, the plies frequently are cut into the shapes and dimensionsdesired for proper placement into the mold. Although the plies may bemolded individually, when objects of greater thickness than thatproduced by the thickness of an individual ply are to be formed, it isconventional to pile the plies in contiguous relationship and to moldthe thicker object therefrom.

However, in such instances, and particularly when two or more plies areso used, and when all the plies are substantially identical, certaindeficiencies in the molded part result. These involve the existence ofcuring seams between the various plies, the seams existing because nomelding of the plies occurs at their contact points. This failure tomeld occurs principally due to unlike curing initiation occurring amongthe individual plies inasmuch as the individual curing fronts proceedinwardly from each ply, or from the heated mold surface, with the resultthat when the individual curing fronts meet at or near the ply edges, acuring seam is created. The presence of such seam, or seams, weakens themolded part principally by providing stressed or slippage surfaces atsuch curing seams along which surfaces shear can occur; thus, the shearstrength of the part is reduced.

Traditionally, compression molding of parts having cross sectionsgreater than 0.25 inch was accomplished by varying the mold cycle time,molding temperature, preheating the SMC preform, or the cut and preparedcharge, or by employing a post cycle cooling schedule. The decrease inmold cycle time, a prime factor in the part processing cost, is anattractive target. Unfortunately, in practice, a compromise is madebetween mold cycle time and the ultimate properties obtained in thethick section. In fact, effective translation of thin sectionproperties, especially tensile and shear strengths, to the thick crosssection is seldom accomplished.

There has now been discovered a solution to this problem. The solutioninvolves employing as the plies a series of moldable polymericcomposities formulated to "co-cure" by arranging the plies such that thematerial with the lowest polymerization initiating temperature ispositioned centermost of the plurality of plies, with plies havingprogressively higher initiating temperatures being positioned outwardlytherefrom in the order of increasing initiating temperature. Properlyevaluated in terms of heat transfer and reaction initiation, whenmolded, all plies begin to cure at substantially the same time, andcuring seams are substantially eliminated.

SUMMARY OF THE INVENTION

In accordance with this invention, molded articles with thicknessesgenerally greater than one-quarter of an inch are produced essentiallywithout internal defects. The method of this invention provides for theselection of specific catalysts or catalyst systems to be utilized ineach ply. The combination of the initiating temperature of the catalystsystems, as obtained from a novel interpretation of the exotherm curvetest, as defined by The Society of the Plastics Industry, Inc., and thedetermination of the thickness of the plies required to accommodate thethermal conductivity characteristics of the specific molding compoundproduces essentially an attainment of concurrent initiation of thecrosslinking polymerizations throughout the plies.

This invention involves a method of molding comprising positioning aplurality of plies comprising crosslinking polymeric compositions withina mold, each of the plies comprising a defined catalyst system, thesecatalyst systems having selected to simultaneously reach theirpolymerization initiation temperature when molded under heat andpressure, and molding the plurality of plies under heat and pressure fora period sufficient to simultaneously raise the catalyst systems totheir polymerization temperatures to crosslink the polymericcompositions and to form a molded object.

That ply having the lowest polymerization initiating temperature ispositioned centermost of the plurality of plies and those pliescomprising catalysts having progressively higher polymerizationinitiating temperatures are positioned outwardly from the centermost ofthe plies.

The method of this invention is applicable to preform wet pourprocessing, pipe over-wrapping, autoclaving, vacuum bag, and all methodsof molding.

The method of this invention can employ material concepts such as thickmolding compounds (TMC), bulk molding compounds (BMC) and sheet moldingcompounds (SMC), and combinations thereof.

The method of this invention is usable in applications wheredifferential flow in a mold is desirable.

In all instances, the invention is applicable to parts of allconfiguration, including those with ribs where the configuration of thepart produces distances from the heat source which would result indifferent catalyst initiating temperatures.

Similarly, the invention is applicable to the use of a single resin orto a plurality of resins, and particularly to those procedures whereinthe thickness of the formed object is greater than 1/8 inch but lessthan 1/2 inch, and preferably greater than 3/8 inch.

In all instances, the objects will be constructed of plies which can beassembled in the shaping medium or external thereto.

The moldable polymeric compositions are mainly described herein in termsof the components which are primary contributors to the determination ofthe reaction initiation temperature, that is, those ingredients such asthe catalyst or catalysts, the unsaturated polyester, the monomer ormonomers, and the thermoplastic modifiers, if any, which influence theinitiation temperature and the reactivity rate; and the fillers, which,together with the former ingredients affect the heat transfer propertiesin a major way. In accordance with known practice, the moldablepolymeric compositions may also include additional appropriateingredients including, for example, inhibitors, fire retardants,lubricants, pigments, mold release agents, wetting agents,reinforcements, thickening agents, and the like.

The method of this invention pertains to any polymeric resin moldableunder heat and pressure, during which molding, polymerization takesplace by catalytic action and which resin has a determinable reactivity.This invention is principally concerned with polymerizable resinouscompositions which can be filled with particulate matter and reinforcedwith glass fiber. Most frequently, these resins are employed in the formof sheet molding compounds or bulk molding compounds and will bereferred to hereinafter as "molding compounds". They will include allunsaturated polyester resinous compositions and modifications thereof asdisclosed in U.S. Pat. No. 3,772,241 to Charles H. Kroekel, whichdisclosure is included herein by reference. Resins also of particularuse are those described in U.S. Pat. No. 3,883,612 to Pratt et al.

By plies or sheets, is meant moldable portions of polymeric compositeswhich can be placed in substantially contiguous relationship over adesired portion of their surfaces, such contiguous relationship beingone wherein heat from one portion is readily transferable to a secondportion by virtue of their contact. While this invention is applicable,in its preferred embodiment, to an odd number of plies, it is alsoapplicable to an even number, including two plies of like or unlikeproperties by the process which is adapted to produce simultaneouscuring from the point of juncture of the plies.

It is a principal object of this invention to provide a method to permitthe molding of a plurality of plies of polyester molding compounds intoan integral molded object without internal flaws. It is a further objectto provide molding cycles for such polyester molding compounds which donot require cumbersome and time consuming mold heating and coolingoperations, or cost prohibitive processing times at extraordinary moldtemperatures. It is an additional object to provide articles ofmanufacture molded in accordance with a novel method which yieldsreproducible high quality and improved strength items.

Other objects and advantages and the broad scope of applicability ofthis invention will be apparent from the following description of theinvention and by reference to the related drawings. It should beunderstood that the description and specific examples which are providedare given by way of illustration only; and that various changes andmodifications, which will become apparent to those skilled in the artfrom the description, are possible within the scope and spirit of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a data plot obtained from the "Standard Exotherm Curve"testing per the SPI Procedure.

FIG. 2 is a data plot obtained when the "Standard Exotherm Curve" testprocedure is performed in the absence of a catalyst.

FIG. 3 is a combination of the data plots from FIGS. 1 and 2.

FIG. 4 is a composite data plot of "Standard Exotherm Curve" testsperformed with preferred catalysts which have different temperatures ofpolymerization initiation.

FIG. 5 is a graphic representation of the thickness-temperaturerelationships which exist throughout the compound at different periodsof time.

DETAILED DESCRIPTION OF THE INVENTION

The method of this invention provides for the selection of catalysts toachieve an essentially simultaneous initiation of the polymerizationreaction throughout the part being molded in spite of the existence ofconcurrent differential temperature zones throughout the part. Thesezones result primarily as a function of the thickness of the part.Equivalently expressed, they result as a function of the distance of aparticular location within the part from the source of external heat.

The major point of novelty of the invention resides in the synergyestablished by the method of the invention for the temperature or heatflow throughout the molding material and the multiple catalystselection.

The temperature or heat flow responds to two main thermal conductivityfactors:

1. The temperature, heating capacity, and thermal conductivityproperties of the mold or heat source surface.

2. The thermal conductivity of the molding compound selected for aparticular application.

Factors 1 and 2, above, represent an infinite number of combinations,particularly since the molding compound, per se, offers an infinitenumber of variations in ingredients and concentrations, but any specificformulation can have its pertinent thermal characteristics readilydetermined by standard physical testing procedures. Preferably, thesedata are obtained on the complete molding compound minus only thecatalyst. A three-dimensional plot of time, temperature, and distancefrom the heat source can then be calculated and constructed for thespecific combination under consideration. For instance, see G. Mengesand H. Derek, Session 23-C, 36th Annual Conference, ReinforcedPlastics/Composites Institute, The Society of the Plastics Industry,Inc., February 16-20, 1981.

The catalysts which can be employed are conventional and can be selectedfrom a host of commercially available products. They may be generallyclassified, but not limited to, aliphatic diacyl peroxides,hydroperoxides, dialkyl peroxides, peroxyketals, aromatic diacylperoxides, peroxyesters, and "azo" catalysts. The proper catalyticinitiating temperatures can be attained by establishing unlikecatalysts, per se, in the different plies, or by unlike quantities ofthe same catalyst or catalyst combinations.

Also includable are promoters conventional within the art, such ascobalt-containing compounds, vanadium-containing compounds,nitrogen-containing compounds, and the like, for example. Promoters, ifused, must be included in the testing to determine the polymerizationinitiating temperatures.

The reactivity of an unsaturated polyester resin is commonly determinedaccording to the procedure designated by the ReinforcedPlastics/Composites Division of the Society of the Plastics Industry,Inc. It was published in the report of the 24th Annual TechnicalConference, 1969, of the Reinforced Plastics/Composites Division, TheSociety of the Plastics Industry, Inc. that: "This method is designedfor use in determining the `exotherm curve` of an uncured polyesterresin, and covers the `standard 180° F. exotherm curve,` other standardexotherm curves, and certain variances which may be required for specialresins or to supply information which is important for specificapplications".

It is a standard practice in the industry to include the followinginformation pertaining to reactivity traits on the data sheets forunsaturated polyester resins:

1. The test bath temperature.

2. The identity and quantity of the catalyst used.

3. The time required for the test resin sample to rise in temperaturefrom 150° F. (65.6° C.) to 10° F. (5.6° C.) above the bath temperature(this time is commonly referred to as "gel time").

4. The time required for the test resin sample to rise in temperaturefrom 150° F. (65.6° C.) to the highest temperature attained during thetest (the peak temperature).

5. The numerical value of the peak temperature attained in degreesFahrenheit.

Referring now to the drawings, shown in FIG. 1 is a typical data plotobtained from the "Standard Exotherm Curve" testing. Point "A" indicatesthe position at which the test sample reached 150° F. (65.6° C.). Point"B," the point at which the test sample had risen to 270° F. (132.2°C.), or ten degrees above the bath temperature of 260° F. (126.7° C.).Point "C," the location at which the test sample had its maximumtemperature.

These data would be reported as follows:

SPI Exertherm Test at 260° F. (126.7° C.) with 1% tertiary butylperoxybenzoate catalyst.

Time from 150° F. (65.6° C.) to 270° F. (132.2° C.)--2.75 minutes.

Time from 150° F. (65.5° C.) to peak--3.25 minutes.

Peak exotherm--495° F. (257.2° C.).

In FIG. 2, a typical data plot is shown where the procedure for theStandard Exotherm Curve Test is performed in the absence of a catalystor similar functioning ingredient. Under these conditions, the testresin sample approaches the bath temperature in an asymptoticrelationship. No standard data points are obtained since the test resinsample does not exceed the bath temperature.

In FIG. 3, the data plots from FIG. 1 and FIG. 2 are combined. Thisprovides a graphic representation of one of the major points of noveltyof this invention, namely, the discovery of the interpretation of thepolymerization initiating temperature and the significance of itsinteraction with the variable heat rate input or temperature profileinduced primarily by distance from a heat source.

The point at which the data curve from FIG. 1 departs from the datacurve from FIG. 2, identified as T_(I), can only be attributed to theadditional thermal input from the initiating crosslinking polymerizationsince in the absence of a catalyst or other initiating ingredient nosuch temperature increase occurs. The temperature at which this pointT_(I) is located is termed the initiating temperature for the catalystor catalyst system used in the test sample. The T_(I) can easily andreadily be located from the data curve for FIG. 1 alone, as T_(I) canalso be defined as the tangent point of the asymptotic portion of thedata plot and the first non-lineate section of the temperature riseabove the projection of the asymptotic rate.

Thus, it has been found that the SPI test procedure may be utilized toobtain novel information not originally intended or apparent in the testas delineated. This provides a simple, practical, and commonly known andavailable mode to accomplish the initial step in the method of thisinvention.

Other and/or more sophisticated practices such as, for example,differential thermal analysis may optionally be employed to yieldevidence of the first initiation of the crosslinking polymerizationwithout departing from the scope of this invention.

With reference again to FIG. 3, special attention is directed to therapid rise in the temperature of the catalyzed test sample which occursgenerally within a half-minute or less after the test sample data curvehas passed through the location of T_(I). The preferred catalystselection possesses not only the proper T_(I) to accommodate thethickness and specific location within the plurality of plies of the plyin which it used, but also generates a rate of crosslinkingpolymerization, as indicated by the locus of the data curve after T_(I),essentially the same as those in the other plies. The synchronisticalrelationship demonstrated in FIG. 4 is thus achieved.

If the thermal expansion forces, and other similar strain inducingelements which can originate during the time span of this rapidtemperature rise, are allowed to occur consecutively throughout aplurality of plies, the defects remedied by the preferred method of thisinvention may occur.

Although numerous combinations and permutations of catalysts andcatalyst systems and ply thicknesses and locations are possible, thepreferred utilization of the method of this invention exercisesprecautionary provisions with regard to the temperatures to which thematerials may be exposed during compounding, during the maturationperiod and inventory storage, as well as the duration of exposure to themold temperature while the molding die is being charged with theassembled plies. For such reasons, the portions of thetime/temperature/distance data plots below about 150° F. (65.6° C.) aregenerally disregarded as impractical for catalyst selection purposes.

As can readily be seen by reference to FIG. 5, the major temperaturedisparity throughout the compound as a function of distance from theheat source occurs shortly after the compound is first exposed to themold heat. This is primrily due to the rapid conduction of heat from themold surface into the adjacent material ply compared to the slowerconduction through the molding compound ply to ply. After this initialperiod, the temperature differential, (Δ_(T)), between the mold surfaceand the adjacent ply is so effectively reduced that the rate oftemperature rise thereafter of the ply adjacent to the mold is analogousto the asymptotic portion of the temperature curve displayed by the testsample in the exotherm curve test as bath temperature is approached. Forthe more inward plies, once a constant heat flow has been establishedthroughout the compound, a relatively uniform temperature gradient willbe maintained across the thickness until reactivity energy begins oruntil the Δ_(T) for a given ply is decreased sufficiently to againinduce an asymptotic relationship. In the absence of polymerization, thetemperature throughout the compound would ultimately rise to that of theheat source, but only after a prohibitively long time. In the practiceof this invention, it is not necessary to achieve temperature uniformlythroughout; the provision for multiple temperature zones is intrinsic inthe method.

To allow sufficient time for charging the assembled plies into the moldin their proper placement alignment; for the mechanical operation of theclosing of the press; and also for the inner plies to attain atemperature above about 150° F. (65.6° C.), which temperature alsoshould be sufficient to reduce the necessary number of temperaturezones, and therefore the necessary number of plies to a practicalamount; it is often desirable that the outer ply have a T_(I)approaching the mold temperature usually of between 225° F. (107.2° C.)and 325° F. (162.7° C.) or even slightly, up to about 10° F. (5.6° C.)above the moldtemperature to accommodate the half-life characteristicsof most commonly used catalysts. The exotherm heat from the adjacent plymay be used to achieve cure initiation essentially simultaneously,particularly since in most instances of the practice of this invention,the outermost plies will have the smallest thickness of all of the pliesused in a given compilation of plies.

The defects arising from current practices are attributable to theinitiation of curing from the outermost, or heat source surface, side ofthe part first and then progressively inward. In the practice of thisinvention, curing is initiated simultaneously throughout, or, in theinstance of an outer ply with a T_(I) above mold temperature, may resultin a curing initiation from the center outwardly. Either curing,simultaneously or from the center outwardly, beneficially results in amolded article essentially free of defects such as curing seams.

The method of this invention first involves the determination of T_(I)values and reactivity rate charts for several catalysts and catalystcombinations when used with the resin selected for the application.While it would be desirable to obtain data to cover the temperaturerange from about 150° F. to about 350° F. (65.6° C. to about 177° C.) inten degree Fahrenheit increments, the range of 200° F. to 300° F. (93°C. to 149° C.) is usually sufficient. Then there is established thetemperature differences which may be expected throughout the part atdifferent periods of time, preferably from about three to about sevenminutes after application of heat, by the determination of the thermalconductivity of the compound. Then there is the generation of atime/temperature/thickness plot similar to that shown in FIG. 5, by anysuitable method such as the direct measurement of temperature changewith time at various positions within a test compound sample withoutcatalyst.

As beforementioned, compound temperatures below about 150° F. (65.6° C.)are generally not desirable, and minimum compound temperatures of about200° F. (93.3° C.) are preferred in order to prevent the number of pliesrequired from being excessive. The time to achieve a minimum compoundtemperature of about 200° F. (93° C.) is selected as a starting point,and the temperature/thickness profile is examined for uniformity. Forexample, if in a given instance, in three minutes after the applicationof heat, a minimum compound temperature of 200° F. (93.3° C.) is reachedat a distance from the heat source equivalent to the centerline of thethickness of the part to be molded, that is, the effective thickness ofthe molding desired, the concurrent temperatures progressing outwardlyto the edge are noted. If an edge temperature of 290° F. (143.3° C.) isfound, for example, nine plies of 10° F. (5.6° C.) differential will berequired; that is, nine different plies which would progressively have200° F. (93.3° C.), 210° F. (98.9° C.), 220° F. (104.4° C.), and so onup to 290° F. (143.3° C.) for their catalyst initiation temperatures.These plies would differ in their thicknesses as was necessary tocoincide with the thickness dimensions equivalent to the ten degree(°F.) temperature spans which exist at the time selected. While thiswould be within the scope of this invention, nine plies might be deemedundesirable.

Additional times such as four minutes, five minutes, and so forth, wouldbe reviewed until a satisfactory compromise between the time and thenumber of plies to be used was obtained. Thus, the method adapts toindividual examples. For each time examined, a different thickness foreach ply and a different number of plies usually results.

The mass to be molded is then formed of a series of plies, a differentply being employed for each temperature differential of about 10° F.(5.6° C.). That is, adjacent plies will differ in their catalystinitiation temperature by about 10° F. (5.6° C.) while the temperaturedifference between the material adjacent the mold, or die, and thematerial located at the center of the part being molded does not exceed50° F. (10° C.), and preferably does not exceed 30° F. (-1.1° C.).

Into each ply is incorporated a polymerization catalyst with aninitiating temperature corresponding to the temperature which that plyis expected to reach at the time selected.

The plies containing the catalyst are then placed in a contiguousrelationship within the mold such that the material with the lowestpolymerization initiating temperature is positioned centermost of theplurality of plies, with the plies having higher initiating temperaturespositioned outwardly therefrom in order of increasing initiatingtemperature, and the mold is closed. Molding is conducted under heat,and pressure if necessary. Inasmuch as the initiation temperature in theplies is reached simultaneously, though at different temperatures, ameld of the plies takes place and an article of improved strength isobtained.

The foregoing will become more apparent and better understood byreferring to the following examples.

EXAMPLE 1

The following unsaturated polyester resin system/filler mixtures wereprepared using a high-speed stirrer equipped with a dispersion blade.

Mixture 1

100 weight parts of an unsaturated polyester resin system made from adicyclopentadiene propylene maleate unsaturated polyester plus apolyvinyl acetate-methyl methacrylate-acrylic acid terpolymer andstyrene.

150 weight parts of a calcium carbonate filler (Snowflake).

1.5 weight parts of tertiary butyl peroxybenzoate catalyst.

Mixture 2

The same as Mixture 1 except for the use of 165 weight parts of calciumcarbonate filler (Snowflake) instead of 150 weight parts.

Mixture 3

The same as Mixture 1 except for the use of 180 weight parts of calciumcarbonate filler (Snowflake) instead of 150 weight parts.

Mixture 4

The same as Mixture 2 except for the use of a mixed catalyst system of1.5 weight parts of Trigonox 29B75 plus 0.5 weight parts of Trigonox KSMinstead of 1.5 weight parts of tertiary butyl peroxybenzoate.

Mixture 5

The same as Mixture 2 except for the use of a mixed catalyst system of0.5 weight parts of Lupersol PDO plus 0.5 weight parts of tertiary butylperoxybenzoate catalyst instead of 1.5 weight parts of tertiary butylperoxybenzoate catalyst.

Aliquot portions of each of the above mixtures were transferred into newclean 19×150 mm test tubes so that three inches of the test mixture, asmeasured from the bottom of the test tube to the top of the testmixture, were in each test tube. This procedure is in compliance withthe procedure specified by the Society of the Plastics Industry, Inc.,for running exotherm curves. Continuing to follow that procedure foreach test run, a thermocouple needle and centering device were insertedinto the test tube containing the mixture materials, the test tube andneedle assembly were placed into a test rack in the heated bath and theexotherm curve obtained. The initiation temperatures were determined bytangency. Three bath temperatures of 260° F., 280° F., and 300° F. (127°C., 138° C., and 149° C.) were used in this testing program as indicatedin the following table. The table shows the averages of replicatetesting.

                  TABLE 1                                                         ______________________________________                                                     Test Bath Temp.,                                                                           Initiation Temp.,                                   Mixture Number                                                                             °F. (°C.)                                                                    °F. (°C.)                             ______________________________________                                        1            260 (126.6)  250 (121.1)                                         2            260 (126.6)  250 (121.1)                                                      280 (137.8)  245 (118.3)                                                      300 (165.5)  245 (118.3)                                         3            260 (126.6)  250 (121.1)                                         4            260 (126.6)  210  (98.9)                                                      280 (137.8)  215 (101.7)                                                      300 (165.6)  210  (98.9)                                         5            260 (126.6)  220 (104.4)                                                      280 (137.8)  230 (110)                                                        300 (148.9)  225 (107.2)                                         ______________________________________                                    

In examining the data in Table 1, it can readily be seen by a comparisonof the initiating temperatures obtained for mixtures Numbers 1, 2 and 3at the 260° F. (127° C.) both temperature that the T_(I) is relativelyunaffected by the weight percent filler contained in the mixture. Also,from a comparison of the T_(I) obtained on mixture Number 2 at the threetest bath temperatures, and similarly on mixtures Numbers 4 and 5, it isapparent that the T_(I) of a given catalyst or catalyst system when usedwith a specific resin system, is essentially unchanged as a function ofthe test bath temperature. By a comparison of the initiatingtemperatures of mixtures Numbers 1, 2 and 3, individually orcollectively, with those for mixtures 4 and/or 5 at a given bathtemperature, it can be established that the T_(I) is a result of theselection of the catalyst or catalyst system. Thus, by theidentification of the initiating temperature as a function of thecatalyst selection, it is possible to generate the information necessaryto select the required catalyst systems to accommodate the temperaturedifferentials anticipated as a result of the "effective thickness" ofthe molding desired, the thermal conductivity of the instance moldingcompound, the tool or heat source properties, and the other pertinenttemperature factors.

The following example demonstrates the preparation of a molding compoundand its use in the method of this invention.

EXAMPLE 2

An unsaturated polyester polymer was synthesized in an ordinary reactionvessel which was equipped with an agitator, a heating means, acondenser, and an inlet for inert gas from the following raw materials:

10.4 gram mols of propylene glycol (791 grams)

3.3 gram mols of isophthalic acid (548 grams)

6.7 gram mols of maleic anhydride (657 grams)

(Isophthalic acid includes commercial material which may contain up toabout 18% of other acid compositions such as terephthalic acid, forexample.)

First Stage

The reactor was first charged with all of the propylene glycol (10.4gram mols; 791 grams) and about 0.1% (2 grams) of dibutyl tin oxideprocessing catalyst based on the total weight (1966 grams) of all of thematerials to be used. The agitation, the heating means, and the inertgas flow were started. When the batch temperature had reached about 300°F. (149° C.), 3.3 gram mols (548 grams) of isophthalic acid were addedto the reactor with constant agitation. Heating under inert gas flow wascontinued until the batch temperature had increased to about 420° F.(216° C.). The batch temperature was maintained at about 420° F. (216°C.) until the acid number had decreased to about 15 or less.

Second Stage

The batch was cooled until a temperature of 300° F. (149° C.) or lowerhad been reached. Then 6.7 gram mols (657 grams) of maleic anhydride andabout 0.01% (0.2 grams) of toluhydroquinone inhibitor based on the totalweight (1966 grams) of all of the materials to be used were added to thereactor with constant agitation. The batch temperature was graduallyincreased to about 410° F. (210° C.) and further processed until an acidnumber of 30 was reached. The batch was then cooled to about 250° F.(121° C.) and diluted with sufficient styrene monomer to yield a resinmix of two weight parts of unsaturated polyester polymer to one weightpart of styrene monomer. A test sample of this final product had aviscosity of 2350 centipoise at 77° F. (25° C.) and an acid number of18.7.

Third Stage

A thermoplastic polymer was synthesized from the following monomers:

71.4 parts of vinyl acetate monomer.

27.4 parts of methyl methacrylate monomer.

1.2 parts of acrylic acid.

About 200 parts of demineralized water were heated to 153° F. (67° C.),and 3% of the above monomer mixture containing about 2 weight parts ofmethyl ethyl ketone (MEK) and 1 weight part of benzoyl peroxide (BPO)were added to the water. The temperature of the aqueous mixture wasmaintained at 153° F. (67° C.) and supplied with a nitrogen gas sparge.The remaining monomers were added to the aqueous mixture over a periodof time and at a steady rate to maintain the batch temperature at about153° F. (67° C.). After the monomers were added, the batch temperaturewas raised to 176° F. (80° C.) and held for one hour. The resultingsuspended thermoplastic polymer was cooled and partially dried bycentrifuging to remove a major portion of the water. The partially driedthermoplastic polymer was then dispersed in styrene monomer and vacuumstripped of the remaining water. The concentration of the thermoplasticpolymer was adjusted to 35 weight parts in 65 weight parts of styrenemonomer and about 0.01% p-benzoquinone inhibitor per 100 weight parts ofthe thermoplastic polymer-styrene mixture was added.

Fourth Stage

The final product of Example 2 was blended at room temperature with thefinal product of Example 3 by charging to a mixing vessel the following:

80 weight parts of Example 2.

20 weight parts of Example 3.

The blend was mildly agitated to form a uniform resinuous system withexcellent phase stability. The viscosity was 2240 centipoises at 77° F.(25° C.).

Fifth Stage

A sample of the above resin is employed to produce an SMC composed of 35weight percent resin paste and 65 percent chopped glass roving.

A volume of compound, complete except for catalyst, of constantcross-section is placed in a mold maintained at 270° F. (132.2° C.).Thermocouples are placed at regular distances across the cross-sectionof the sample and temperature determinations are made with the followingresults after seven minutes.

    ______________________________________                                        Distance From                                                                 Mold Surface A, inches                                                                        Temperature, °F. (°C.)                          ______________________________________                                        0.16            260 (126.6)                                                   0.33            256 (124.4)                                                   0.5             252 (122.2)                                                   0.6             248 (120)                                                     0.83            244 (117.7)                                                   ______________________________________                                    

Based upon these data, five plies of indicated thickness, are preparedand into each is incorporated a polymerization catalyst having thefollowing initiating temperatures.

    ______________________________________                                                               Catalyst                                                                      Kick-Off                                               Ply Number                                                                              Ply Thickness, in.                                                                         Temperature, °F. (°C.)                   ______________________________________                                        1         0.110        266 (130)                                              2         0.35         258 (125.6)                                            3         0.74         249 (120.6)                                            4         0.35         258 (125.5)                                            5         0.110        266 (130)                                              ______________________________________                                    

The five plies are stacked in the mold in continguous relationship inthe order of initiating temperature decreasing inwardly from the mold.The mold is closed and the plies are molded at a temperature of 270° F.(132° C.) and 1100 psig for a period of seven minutes and heldthereafter for thirteen minutes for a total of twenty minutes.

Inspection of the molded part shows essentially no seams between theplies, indicating substantially simultaneous polymerization within allthree plies.

EXAMPLE 3

The SMC formulation was a randomly-oriented chopped glass in a polyestermatrix. No filler was used because of the high-glass loading.

A one-inch constant thickness disc 21 inches in diameter was used formolding large volume parts. The thickness of the disc was about one inchand was comprised of an odd number of plies which were molded inoil-heated mold halves coated with a mold release agent.

Eighteen layers were employed. These layers were arranged such that theinnermost six layers consisted of the lowest T_(I) compound and theouter three layers on each side of the stack were the highest T_(I)material. Three intermediate T_(I) layers were placed between theinnermost and outermost plies.

While 18 layers were employed, as a practical matter of grouping, theseequated to but 3 plies as concerns catalyst type or concentrations.

Ply compositions were as follows:

    ______________________________________                                                          Ply, weight parts                                                                   Low    Intermediate                                                                           High                                  Function                                                                              Component       T.sub.I                                                                              T.sub.I  T.sub.I                               ______________________________________                                        Resin   Resin of Ex. 2  100    100      100                                   Catalyst                                                                              t-butyl peroxybenzoate                                                                        1.7    0.6      0.5                                   Initiator                                                                             t-butyl peroctoate                                                                            0.7    0.1      0                                     Thickener                                                                             Magnesium oxide 5      5        5                                     ______________________________________                                    

Such methods of molding have produced materials of superior tensileproperties. The following data is presented relative to structural sheetmolding composed of chopped glass in polyester resin for one-inch andtwo-inch reinforced molded objects.

                  TABLE I                                                         ______________________________________                                        Structural Sheet Molding Compound* Properties                                                  1-Inch                                                                              2-Inch                                                 ______________________________________                                        Tensile Strength (ksi)                                                                           36.7    46.5                                               Tensile Modulus (msi)                                                                            2.40    2.44                                               Tensile Elongation 2.27    2.49                                               Failure (%)                                                                   Poisson's Ratio    0.26    0.26                                               Density (lb/in.sup.3)                                                                            0.064   0.062                                              ______________________________________                                         *65 weight percent chopped glass (one or two inch chopped length) and 35      weight percent polyester based resinpaste.                               

It will be evident from the foregoing that various modifications of thisinvention can be employed. Such, however, are considered within thescope of the invention.

I claim:
 1. A method for molding a plurality of polymeric plies to forma composite article, comprising:(a) disposing a plurality of compatiblepolymeric plies in a mold in contact relationship, at least same of saidplies having catalyst systems with polymerization initiatingtemperatures different from polymerization initiation temperatures ofcatalyst systems of others of said plies, said initiation temperaturesbeing such that when all of said plies are suitably heated within saidmold, all of the catalyst systems reach their several differentinitiation temperatures substantially simultaneously; and (b) moldingsaid plurality of plies under pressure and heating for a period and in amanner such as to raise said polymeric plies and said catalyst systemsto their several different polymerization initiation temperatures toinformly crosslink said polymeric plies and form said polymeric article.2. The method of claim 1 in which said plies are arranged in contiguousrelationship.
 3. The method of claim 1 in which said crosslinkingpolymeric composition comprises an unsaturated polyester, athermoplastic polymer and a monomer.
 4. The method of claim 2 in whichsaid catalysts are selected from the group consisting of aliphaticdiocyl peroxides, hydroperoxides, dialkyl peroxides, peroxyketals,aromatic diacyperoxides, peroxyesters and azo catalysts.
 5. The methodof claim 1 in which said composition comprises promoters, glass fibers,fillers and colorants.
 6. The method of claim 1 in which thepolymerization initiation temperatures of adjacent plies differ by about10° F.
 7. The method of claim 1 in which an odd number of plies arepositioned in said mold.
 8. The method of claim 1 in which an evennumber of plies are positioned in said mold.
 9. The method of claim 1 inwhich each ply comprises a plurality of layers.
 10. The method of claim1 in which said plies differ in catalyst type.
 11. The method of claim 1in which said plies differ in catalyst concentration.
 12. The productformed by the method of claim
 1. 13. The method of claim 1 in which saidplies are assembled in successive face to face contact by arranging theplies such that the material with the lowest polymerization initiatingtemperature is positioned centermost of the plurality with plies havingprogressively higher initiating temperatures being positioned outwardlytherefrom in order of increasing initiating temperature and heating theassembly from the exterior of said assembly so that said plies arecaused to cure simultaneously.
 14. The method of claim 1 in which thatply having the lowest polymerization initiating temperature ispositioned centermost of said plurality of plies.
 15. The method ofclaim 16 in which plies comprising catalysts having progressively higherpolymerization initiating temperatures are positioned outwardly from thecentermost of said plies.